Skin has a tendency to become dry upon exposure to air and sun; the loss of water at the skin surface also results in a loss of water in the stratum corneum. For this reason, it is important for the skin to be well moisturized and not to suffer a loss of water which withers skin, and thus causes its premature ageing, drying and even desquamation. Thus, in the cosmetics field, it is common to incorporate into compositions used as moisturizing agents hygroscopic substances which bring about a rehydration of the skin by uptake of atmospheric water and by retention of the water in the skin.
Among the many moisturizing agents, the most commonly used are polyols, and especially glycerol which is a water trap on account of its hydroxyl groups; one molecule of glycerol takes up six molecules of water. Furthermore, glycerol is not very bulky, enabling it to penetrate into the skin. See the paragraph on glycerine in The Principles and Practice of Modern Cosmetics by R. G. Harry, 1963, Volume II pages 202 to 205 incorporated herein by reference. The only drawback with glycerol is that it gives a sticky nature to compositions which contain large amounts of it.
It is also known to use hydroxy acids and salts thereof, and especially lactic acid and sodium lactate, the latter being one of the components of the NMF (Natural Moisturizing Factor) present in the skin; indeed, it is thought that lactic acid or the salt thereof modifies the spatial conformation of the proteins in the stratum corneum. As a result, it improves the suppleness and the elasticity of the skin. See the article by M. Rieger, Cosmetics & Toiletries, 1992, Vol. 107, pp. 89-90 incorporated herein by reference. Unfortunately, hydroxy acids and salts thereof have the drawback of being difficult to incorporate into a cosmetic and/or dermatological composition, especially when it is desired to use them in large amounts, because they are then incompatible with most of the gelling agents usually used to stabilize such compositions. This incompatibility is reflected in the destabilization of compositions which contain them. In the case of an emulsion, a separation of the aqueous and oily phases takes place, and in the case of a gel, it breaks.
In order to overcome this problem, it is possible either to incorporate hydroxy acids and salts thereof in very small amounts, or to use them in very specific vehicles which are compatible, which, in both cases, limits the freedom of formulation. Thus, lactic acid or sodium lactate are still used with the same types of gelling agent and of emulsifying agent, and compositions of the same texture, usually greasy and waxy, are thus still obtained. It is thus very difficult to use hydroxy acids and salts thereof in just any type of composition. In particular, it is not possible to obtain milks and lotions.